Regulación Baja Tensión Transformadores 25kVA

DIN 1054:2005-01 section 6.1.3 also states the following: “The interaction of subsoil and struc- ture is to be taken into account if the stiffness of the structure in conjunction with the stiffness of the subsoil causes a considerable redistribution of the forces transferred to the soil.” Further- more, the DIN standard states that serviceability is to be verified with the characteristic actions and resistances. In doing so, the same structural system shall apply as was used for determin- ing the internal forces or action affects at limit state LS 1B, and variable actions shall only be taken into account if they cause irreversible displacements or deformations. The DIN standard does not mention any particular method of calculation in conjunction with the verification of serviceability.

With respect to the use of methods of calculation for taking into account the soil-structure in- teraction, EC7 comments: “. . . problems of soil-structure interaction analyses should use stress- strain relationships for ground and structural materials and stress states in the ground that are sufficiently representative, for the limit state considered, to give a safe result”.

Verification of serviceability for wall-type retaining structures can be carried out with the coef-

ficient of subgrade reaction method or the finite element method (FEM); see, for example,

the excavation recommendations of WEISSENBACH (2003). FEM offers the advantage of a comprehensive modelling of the soil-structure interaction because the material behaviour of structures and soils plus the force transfer to the soil-structure boundary surfaces can be taken into account.

Like in other civil engineering disciplines, FEM has become a standard approach in the veri- fication of serviceability for geotechnical structures. One reason for this is the user-friendly software, another is the progress in the field of material models for soils. However, it should be remembered that in comparison to structural problems, geotechnical problems are based on a much less secure database.

The use of FEM for retaining wall structures is covered in chapter 8.

6.9

Overall stability

The overall stability of changes of level in the terrain in the meaning of an embankment or step in the ground is dealt with in DIN 1054:2005-01, with reference to E DIN 4084. An allocation to geotechnical categories is also necessary for this verification.

For a sheet pile structure, an adequate margin of safety against ground failure is assured when the failure mechanisms possible with this type of wall and the possible critical construction

conditions do not exceed the limit state conditions according to E DIN 4084 with the partial

safety factors for limit state LS 1C given in table 6.3:

Ed≤ Rd (6.64)

where

Ed design value of resultant action effect parallel to slip plane, or design value of mo-

ment of actions about the centre of the slip circle

R_d design value of resistance parallel to slip plane, or design value of moment of resis- tances about the centre of the slip circle

130 CHAPTER 6. DESIGN OF SHEET PILE WALLS

The actions and resistances are calculated as follows: Ed = r ·



i

(Gd,i+ Qd,i) · sin ϑi+



Ms (6.65)

Rd = r ·



i

(Gd,i + Qd,i− ud,i· bi) · tan ϕd,i + ci,d· bi

cos ϑi+ μ · tan ϕd,i· sin ϑi

(6.66)

with the design values of the shear parameters

tan ϕd = tan ϕk

γ_ϕ cd =

ck

γ_c (6.67)

The calculation is carried out iteratively by choosing a degree of utilisation for μ and recalcu- lating according to

μ = Ed

R_d (6.68)

E DIN 4084 explains how to take account of the loadbearing effect of tension members, anchors and piles when checking the overall stability.

See Fig. 6.23 for the general geometrical definition of the aforementioned variables.

Legend

1 Slip circle divided into slices 2 Groundwater level

3 Outer water level 4 Non-permanent loading 5 Stratum boundary 6 Numbering of slices

6.9. OVERALL STABILITY 131

Example 6.16 Verification of overall stability of sheet pile wall to DIN 1054:2005-01 / E

DIN 4084

Verification of an adequate margin of safety against ground failure is carried out for the system shown in example 6.10 and 6.11. The centre of the slip circle is to be determined iteratively so that the critical slip circle with the smallest margin of safety is considered.

Sketch of system: 0.0 5 0.10 0.1 0 0.15 0.15 0.20 0.25 0.3 0 0.3 5 0.4 0 0.45 0.50 5 5 1 2 3 4 5 6 7 Coordinates x_m = - 2.0 m y_m = 0.0 m r = 14.0 m q_k = 20 kN/m² g_k = 10 kN/m² 9.0 m 2.30 m 2.35 m 0.45 m 2.5 m 5.0 m 5.35 m 5.55 m 1.55 m 3.0 m 2.0 m 2.0 m 4.6 m 5.6 m 1.4 m 0.4 m γ/γ' = 18/8 kN/m³ ϕ = 30˚ γ' = 9 kN/³ ϕ = 25˚ c = 10 kN/m² γ' = 10 kN/m³ ϕ = 32.5˚

According to E DIN 4084, the method of slices after BISHOPshould be used for calculating the margin of safety against failure of the ground in stratified soils. In doing so, the slip circle should be formed as accurately as possible by polygonal slices with vertical contact faces. In this example, 7 slices have been chosen. The vertical loads plus the geometry are then determined for each slice, and with the help of eq. 6.66 and 6.67 the variables Edand Mdare determined according to eq. 6.64.

i Gk,i γG Qk,i γQ Gd,i+ Qd,i ϑ l ϕd cd Ed Rd

− kN/m − kN/m − kN/m ◦ m ◦ kN/m2 kN/m kN/m 1 2 3 4 5 6 7 8 9 10 11 12 1 28.8 1.0 0.0 1.3 28.8 -42.6 3.4 27.0 0.0 -263 446 2 173.8 1.0 0.0 1.3 173.8 -25.2 5.5 27.0 0.0 -1036 1957 3 260.8 1.0 0.0 1.3 260.8 -4.8 5.4 27.0 0.0 -306 1973 4 700.9 1.0 40.0 1.3 752.9 18.6 4.9 27.0 0.0 3362 4667 5 653.8 1.0 0.0 1.3 653.8 44.7 7.9 27.0 0.0 6438 4025 6 105.7 1.0 0.0 1.3 105.7 65.0 3.3 20.5 8.0 1341 1202 7 16.4 1.0 0.0 1.3 16.4 84.3 4.0 24.8 0.0 228 157  9766 14427 The anchor force was neglected in the slip circle calculation because its line of action passes approximately through the centre of the slip circle.

The margin of safety against ground failure calculated iteratively results in

μ = 0.68 = 9766 14427 ≤ 1.0

Chapter 7

Ground anchors

7.1

Types of ground anchors

Irrespective of the type of ground anchors, we distinguish between two basic anchor functions: temporary anchors with a maximum service life of two years, and permanent anchors which first and foremost must satisfy higher demands regarding corrosion protection.

Ground anchor types are classified as follows with respect to their form of construction:

• Round steel tie rods (laid in the ground) with anchor wall/plate • Grouted anchors to DIN EN 1537

• Driven anchor piles

• Driven pile with grouted skin • Vibratory-driven grouted pile • Micropiles (diameter ≤ 300 mm) • Jet-grouted piles

• Retractable raking piles

7.1.1 Round steel tie rods

Round steel tie rods consist of tension bars that are laid horizontally in the ground and terminate at an anchor wall or anchor plate. The loadbearing capacity of these anchors may be limited by the passive earth pressure that can be mobilised in front of the anchor wall/plate. Both the threaded and the plain parts of the tie rod must be checked. For practical reasons, the tie rods should not be smaller than 11/2in. Please refer to EAU 2004 sections 8.2.6.3 (R 20) and 9.2.3.3 for further information.

134 CHAPTER 7. GROUND ANCHORS

7.1.2 Grouted anchors

Grouted anchors consist of a steel tension bar surrounded by a layer of injected grout. The tensile forces are either transferred continuously from the tie rod to the grout (composite anchor) or they are transferred via a pressure pipe embedded in the injected grout (duplex anchor). Both systems transfer the forces into the soil by way of skin friction. The steel tension bar must be able to deform freely in a sleeve so that the grouted anchor can be prestressed. Threaded bars or wire tendons can be used as the tension members.

Grouted anchors are normally installed by drilling with or without water-jetting. The sleeve is inserted to the right depth and the steel tension member installed. As the sleeve is withdrawn, the cement mortar is injected under pressure. Above the intended layer of grout, the drilled hole is cleared of mortar and filled in order to avoid a force “short-circuit” between the wall and the layer of grout. A special re-injection process can be used to break apart a layer of grout that has already hardened and press it against the soil, which enables much higher skin friction values to be mobilised. Grouted anchors are covered by DIN EN 1537.

7.1.3 Driven anchor piles

Various steel sections and precast concrete piles can be used as anchor piles. Anchor piles carry the tensile forces on their surface by way of skin friction. They are frequently encountered in quay wall structures in which high tensile forces occur (see Fig. 7.1). In such cases, steel piles enable a straightforward welded connection between pile and retaining wall structure.

Driven piles at shallow angles are guided by leaders. Slow-action hammers are preferred to rapid-action devices (EAU 2004 section 9.5.2). In the case of raking anchor piles, settlement due to backfilling, relieving excavations or the installation of further piles behind the sheet pile wall can lead to loads at an angle to the axis of the pile. The additional deformations cause an increase in the stresses in the pile which in some circumstances means that the maximum axial force may not occur at the head of the pile but instead behind the sheet pile wall (see MARD- FELDT, 2006). This must be taken into account when designing the piles and the connection to the wall. For further information regarding the design and driving of piles, please refer to EAU 2004 section 9.5 (R 16).

7.1.4 Driven pile with grouted skin

The driven pile with grouted skin consists of a steel section with a special driving shoe which cuts a prismatic void in the soil during driving. Cement mortar is injected into this at the same time as driving. This creates a bond between pile, cement and soil which enables skin friction values to be achieved that are 3 to 5 times higher than a non-grouted pile (EAU 2004 section 9.2.1.3).

7.1.5 Vibratory-driven grouted pile

The toe of the vibratory-driven grouted pile, a steel H-section, is widened with welded web and flange plates. As the pile is vibrated into the ground, these displacement elements create a void

7.1. TYPES OF GROUND ANCHORS 135

equal in size to the thickness of the plates, into which a cement suspension is injected in order to increase the skin friction of the pile. Please refer to EAU 2004 section 9.2.1.4 for further information.

Raking pile PSt 600 / 159 l = 45.00 m Double pile section

PSp 1001 l = 41.00 m Intermediate pile section PZa 675 / 12 a = 2.31 m

Figure 7.1: Driven anchor piles, CT IV container terminal, Bremerhaven

7.1.6 Micropiles (diameter

≤ 300 mm)

The term micropile covers various non-prestressed pile types with a small diameter which trans- fer the tensile forces into the soil by way of skin friction. These include, for example, self-boring micropiles to DIN 4128 or DIN EN 14199, tubular grouted piles, grouted in situ concrete piles and composite piles. The self-boring micropile is constructed like a ground anchor, with the full length of the pile embedded in mortar, which improves the corrosion protection.

In the case of the TITAN micropile to DIN EN 14199, which belongs to the group of tubular grouted piles, a ribbed steel tube serves as tension member, lost drilling rod and injection pipe. The tip of the rod includes a radial jet with which the soil can be cut away and at the same time filled with mortar. It is not necessary to install the tension member and withdraw the casing with this system. In soft soils, ground with a high water table or weathered rock, where the drilled hole would collapse, a casing is unnecessary because a bentonite slurry can be used to keep the hole open. This increases the efficiency of the installation work by about 2 to 3 times over the method with a casing in the hole.

The dynamic injection of cement slurry directly after drilling results in a mechanical interlock between layer of grout and soil. The good shear bond means that only minor deformations of the pile head ensue under service loads. EAU 2004 section 9.2.2 contains further information.

136 CHAPTER 7. GROUND ANCHORS

7.1.7 Jet-grouted piles

Jet-grouted piles are bored piles with an enlarged toe. A steel section acts as the tension member. At the base of the pile, the soil is cut away with a high-pressure water jet and mixed with mortar.

7.1.8 Retractable raking piles

Retractable raking piles are used behind quay walls built in water. A steel section welded to an anchor plate forms the tension element. The connection between the head of the pile and the wall still permits rotation. The pile is fixed to the wall while suspended from a crane and subsequently lowered into place, rotating about its fixing point. The resistance of this construction is first activated upon backfilling the wall and is made up of the horizontal passive earth pressure plus the vertical soil weight acting on the anchor plate. EAU 2004 section 9.2.3.1 contains further information.